Field of the Invention
The present invention relates to pneumatic tires that are to be mounted to two-wheeled automotive vehicles. More specifically, the present invention relates to improvement of treads of tires.
Description of the Related Art
In cornering of a two-wheeled automotive vehicle, a centrifugal force is applied to the two-wheeled automotive vehicle. In the cornering, a cornering force is necessary. The cornering force matches the centrifugal force. In the cornering, a rider leans the two-wheeled automotive vehicle toward an inner side. By the two-wheeled automotive vehicle being leaned, the cornering of the two-wheeled automotive vehicle is achieved. In order to facilitate the cornering, a tire for use in two-wheeled automotive vehicles includes a tread having a small radius of curvature. In straight running, the tread contacts with the ground at a center region of the tread. On the other hand, in cornering, the tread contacts with the ground at a shoulder region of the tread. A tire in which a center region and a shoulder region function according to their roles, respectively, is disclosed in JP2005-271760.
In this tire, a hardness of a crosslinked rubber in a center region of a tread is high, and a hardness of a crosslinked rubber in shoulder regions of the tread is low. Thus, in straight running, the tread contacts with the ground at the center region having a high hardness. The tire is excellent in steering stability in straight running. In cornering, the tread contacts with the ground at the shoulder region having a low hardness. The tire is excellent in grip performance in cornering.
The grip performance is influenced by a temperature of a tire. In the beginning of running, the temperature of a tire is low. In a tire that is flexible in a shoulder region, a high grip performance can be obtained in a state where the temperature of the tire is low. A hardness of a crosslinked rubber in the shoulder region is low. In the tire, the crosslinked rubber in the shoulder region becomes more flexible in a state where the temperature of the tire is high. This flexible state leads to reduction of stiffness in the shoulder region. The reduction of stiffness deteriorates steering stability. Further, the reduction of stiffness reduces wear resistance in the shoulder region of the tread. It is not easy to favorably obtain both a grip performance in the beginning of running in which the temperature of a tire is low, and a grip performance in a state where the temperature of the tire is sufficiently high after the beginning of running.
An object of the present invention is to make available a tire, for use in two-wheeled automotive vehicles, which exhibits an excellent grip performance in cornering from the beginning of running.